Update the icon of a parent item without children in wxDataViewCtrl.

Update the icon of a parent item without children in wxDataViewCtrl.

Ensure that a parent item that doesn't have any children any more isn't left
with a "-" expander icon, it can't be collapsed any more but only expanded
again (possibly adding children under it dynamically). This results in better
behaviour in e.g. the last page of the dataview sample where the container
item remained with a "+" icon even after its both children were deleted.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72325 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775